There are manufacturing applications in which it could be useful to weld members of dissimilar metal compositions to fabricate, for example, relatively light-weight articles. For example, in the manufacture of automotive vehicle body parts it might be desired to bond an aluminum alloy reinforcing strut to a magnesium alloy panel. Often, such dissimilar metal members are difficult to join by conventional joining techniques such as fusion welding processes because they form massive, brittle intermetallic compositions that weaken the joint. It is contemplated that such dissimilar metal parts might be joined using friction stir welding practices.
In friction stir welding a rotating tool with an axial probe and shoulder is pressed into a surface of an assembly of metal workpieces. The rotating probe and shoulder engage the workpieces at a welding site. The frictional heat and continued pressure on the probe and shoulder temporarily soften, plasticize, and mix material in engaged portions of the workpieces. When the rotating tool is pressed generally perpendicularly into a spot on the workpieces and then retracted, a friction stir spot weld is formed. The friction stir tool may be retracted and moved and successively engaged along the surface of one or more workpieces to form a series of friction stir spot welds. When the rotating tool is pressed into a workpiece surface and moved in the surface, a friction stir linear weld or seam weld may be formed. Similarly, the friction stir tool may be moved along an interface of abutting edges of two or more workpieces to form a friction stir butt weld. Collectively, these various weld patterns are referred to as friction stir welding (FSW). FSW may include friction stir spot welding (FSSW).
Where the composition of the metal pieces to be joined yields a suitable weld zone, good joint strengths may be obtained. When some dissimilar metals are joined with FSW, the formation of brittle, low melting point intermetallic materials in the weld zone may yield weak or brittle weld bonds. This may happen when, for example, it is desired to join a magnesium alloy member to an aluminum alloy part.
It is an object of this invention to provide a method of achieving strong friction stir weld bonds between workpieces of dissimilar metal compositions such as, for example, between magnesium alloy workpieces and aluminum alloy workpieces.